In an immunodiffusion test, a strip of filter paper containing diphtheria antitoxin is placed on a solid culture medium. Then bacteria are streaked perpendicular to the filter paper. If the bacteria are toxigenic,
a. the filter paper will turn red.
b. a line of antigen–antibody precipitate will form.
c. the cells will lyse.
d. the cells will fluoresce.
e. none of the above

Verified step by step guidance

1. Understand the purpose of the immunodiffusion test, which is to detect the presence of specific antigens or antibodies through the formation of a precipitate.
2. Recognize that the test involves a filter paper with diphtheria antitoxin, which will interact with any diphtheria toxin produced by the bacteria.
3. Consider the mechanism of antigen-antibody interaction, where a visible line of precipitate forms when the antigen (toxin) and antibody (antitoxin) meet and react.
4. Evaluate the options given: a red color change, cell lysis, fluorescence, or precipitate formation, and relate them to the expected outcome of an antigen-antibody reaction.
5. Conclude that the formation of a line of antigen-antibody precipitate is the expected result if the bacteria are toxigenic, as this indicates the presence of the diphtheria toxin.

Key Concepts

Immunodiffusion Test

An immunodiffusion test is a laboratory technique used to detect the presence of specific antigens or antibodies in a sample. In this test, antigens and antibodies diffuse through a gel or solid medium, allowing for the formation of visible precipitin lines where they meet. This method is particularly useful for identifying toxigenic bacteria by observing the interaction between bacterial products and specific antibodies.

Toxigenic Bacteria

Toxigenic bacteria are microorganisms that produce toxins, which are harmful substances that can cause disease in hosts. In the context of the immunodiffusion test, if the bacteria are toxigenic, they will produce toxins that interact with the diphtheria antitoxin present on the filter paper, leading to a specific immune response. This characteristic is crucial for diagnosing infections caused by such bacteria.

Antigen-Antibody Precipitation

Antigen-antibody precipitation occurs when soluble antigens react with specific antibodies, forming a visible precipitate. This reaction is a key principle in immunological assays, including the immunodiffusion test. The formation of a precipitin line indicates a successful interaction between the antigens produced by the bacteria and the antibodies in the test, confirming the presence of toxigenic bacteria.